1. Field of the Invention
The present invention relates to an automatic gain control device.
2. Description of the Related Art
In a conventional automatic gain control device having a filter in a control loop, an output signal of the conventional automatic gain control device may not be converged due to a delay of response characteristics thereof caused by a signal delay from the filter. In order to solve such a problem, a control method has been proposed in which, if a signal level has a sudden variation, a control signal generator for generating a control signal from the signal is switched to another control signal generator having an increased time constant to improve a delay of the control signal caused by the filter, and simultaneously, control information is copied between the control signal generators, such that an output signal of an automatic gain control device has no variation in level (for example, see Japanese Patent Publication No. 3240458).
Also, in the conventional automatic gain control device having the filter in the control loop, if a level control of an automatic gain control is performed by in-band signals of the filter, the automatic gain control is converged in conformity with levels of the in-band signals of the filter. Therefore, if out-of-band signals of the filter are greater than in-band signals of the filter, saturation caused by the out-of-band signals of the filter may occur in an amplifier disposed at a front end of the filter. Accordingly, a control method has been proposed in which amplifiers are cascade-connected through a filter and an automatic gain control is separately carried out with respect to each amplifier disposed at front and rear portions of the filter (for example, see Japanese Patent Publication No. 3086060).
Although the above-mentioned Japanese Patent Publication No. 3240458 discloses a technique capable of improving a delay of response characteristics caused by a signal delay from a filter and converging an output signal of an automatic gain control device, when a level control of an automatic gain control is carried out by in-band signals of the filter, this technique was not able to solve a problem in that saturation may occur in an amplifier disposed at a front end of the filter caused by out-of-band signals of the filter, if out-of-band signals of the filter are greater than in-band signals of the filter.
In addition, although the above-mentioned Japanese Patent Publication No. 3086060 can adjust a signal level at a front end of a filter by tracking variations of levels of out-of-band signals of the filter, if an automatic gain control for the amplifier disposed at the front end of the filter responds at high speed to according to variation of out-of-band signals of the filter, unnecessary variation may occur in the in-band target signals of the filter.
More specifically, if the automatic gain control for the amplifier disposed at the front end of the filter responds at high speed according to the variations of the signals, the in-band target signals of the filter may be subject to unnecessary variation, such that the automatic gain control for the amplifier disposed at the rear end of the filter must follow the variations of the target signals and thus a high-speed response characteristic is required. However, if the automatic gain control with respect to the target signals is carried out with the high-speed response, there is a problem in that signals may be modulated due to the automatic gain control and signal distortion may occur because of an amplitude compression action of the automatic gain control.
In the technique disclosed in the above-mentioned Japanese Patent Publication No. 3086060, therefore, the automatic gain control for the amplifier disposed at the front end of the filter must be performed with a low-speed response according to the variations of out-of-band signals of the filter. In this case, it is necessary to accept noise, which is generated by the out-of-band signals of the filter from low response characteristics of the automatic gain control. Otherwise, if the automatic gain control is performed at high speed with respect to both amplifiers disposed at front and rear ends of the filter, the technique according to the Japanese Patent Publication No. 3086060 must permit distortion of the target signals due to the automatic gain control for the amplifier disposed at the rear end of the filter. That is, in either case, communication quality cannot be improved.